Real Time Dashboards¶
Citus provides real-time queries over large datasets. One workload we commonly see at Citus involves powering real-time dashboards of event data.
For example, you could be a cloud services provider helping other businesses monitor their HTTP traffic. Every time one of your clients receives an HTTP request your service receives a log record. You want to ingest all those records and create an HTTP analytics dashboard which gives your clients insights such as the number HTTP errors their sites served. It’s important that this data shows up with as little latency as possible so your clients can fix problems with their sites. It’s also important for the dashboard to show graphs of historical trends.
Alternatively, maybe you’re building an advertising network and want to show clients clickthrough rates on their campaigns. In this example latency is also critical, raw data volume is also high, and both historical and live data are important.
In this technical solution we’ll demonstrate how to build part of the first example, but this architecture would work equally well for the second and many other use-cases.
Running It Yourself¶
There will be code snippets in this tutorial but they don’t specify a complete system. There’s a github repo with all the details in one place. If you’ve followed our installation instructions for running Citus on either a single or multiple machines you’re ready to try it out.
The data we’re dealing with is an immutable stream of log data. We’ll insert directly into Citus but it’s also common for this data to first be routed through something like Kafka. Doing so has the usual advantages, and makes it easier to pre-aggregate the data once data volumes become unmanageably high.
We’ll use a simple schema for ingesting HTTP event data. This schema serves as an example to demonstrate the overall architecture; a real system might use additional columns.
-- this is run on the master CREATE TABLE http_request ( site_id INT, ingest_time TIMESTAMPTZ DEFAULT now(), url TEXT, request_country TEXT, ip_address TEXT, status_code INT, response_time_msec INT ); SELECT master_create_distributed_table('http_request', 'site_id', 'hash'); SELECT master_create_worker_shards('http_request', 16, 2);
When we call master_create_distributed_table
we ask Citus to hash-distribute
http_request using the
site_id column. That means
all the data for a particular site will live in the same shard.
When we call master_create_worker_shards we tell Citus to create 16 shards and 2 replicas of each shard (for a total of 32 shard replicas). We recommend using 2-4x as many shards as CPU cores in your cluster. Using this many shards lets you rebalance data across your cluster after adding new worker nodes.
Using a replication factor of 2 means every shard is held on multiple workers. When a worker fails the master will prevent downtime by serving queries for that worker’s shards using the other replicas.
In Citus Cloud you must use a replication factor of 1 (instead of the 2 used here). As Citus Cloud uses streaming replication to achieve high availability maintaining shard replicas would be redundant.
With this, the system is ready to accept data and serve queries! We’ve provided a data ingest script you can run to generate example data. Once you’ve ingested data, you can run dashboard queries such as:
SELECT date_trunc('minute', ingest_time) as minute, COUNT(1) AS request_count, SUM(CASE WHEN (status_code between 200 and 299) THEN 1 ELSE 0 END) as success_count, SUM(CASE WHEN (status_code between 200 and 299) THEN 0 ELSE 1 END) as error_count, SUM(response_time_msec) / COUNT(1) AS average_response_time_msec FROM http_request WHERE site_id = 1 AND date_trunc('minute', ingest_time) > now() - interval '5 minutes' GROUP BY minute;
The setup described above works, but has two drawbacks:
- Your HTTP analytics dashboard must go over each row every time it needs to generate a graph. For example, if your clients are interested in trends over the past year, your queries will aggregate every row for the past year from scratch.
- Your storage costs will grow proportionally with the ingest rate and the length of the queryable history. In practice, you may want to keep raw events for a shorter period of time (one month) and look at historical graphs over a longer time period (years).
You can overcome both drawbacks by rolling up the raw data into a pre-aggregated form. Here, we’ll aggregate the raw data into a table which stores summaries of 1-minute intervals. In a production system, you would probably also want something like 1-hour and 1-day intervals, these each correspond to zoom-levels in the dashboard. When the user wants request times for the last month the dashboard can simply read and chart the values for each of the last 30 days.
CREATE TABLE http_request_1min ( site_id INT, ingest_time TIMESTAMPTZ, -- which minute this row represents error_count INT, success_count INT, request_count INT, average_response_time_msec INT, CHECK (request_count = error_count + success_count), CHECK (ingest_time = date_trunc('minute', ingest_time)) ); SELECT master_create_distributed_table('http_request_1min', 'site_id', 'hash'); SELECT master_create_worker_shards('http_request_1min', 16, 2); -- indexes aren't automatically created by Citus -- this will create the index on all shards CREATE INDEX http_request_1min_idx ON http_request_1min (site_id, ingest_time);
This looks a lot like the previous code block. Most importantly: It also shards on
site_id and it also uses 16 shards with 2 replicas of each. Because all three of
those match, there’s a 1-to-1 correspondence between
http_request shards and
http_request_1min shards, and Citus will place matching shards on the same worker.
This is called colocation; it makes queries such as joins faster and our rollups possible.
In order to populate
http_request_1min we’re going to periodically run the equivalent
of an INSERT INTO SELECT. Citus doesn’t yet support INSERT INTO SELECT on distributed
tables, so instead we’ll run a function on all the workers which runs INSERT INTO SELECT
on every matching pair of shards. This is possible because the tables are colocated.
-- this function is created on the workers CREATE FUNCTION rollup_1min(p_source_shard text, p_dest_shard text) RETURNS void AS $$ BEGIN -- the dest shard will have a name like: http_request_1min_204566, where 204566 is the -- shard id. We lock using that id, to make sure multiple instances of this function -- never simultaneously write to the same shard. IF pg_try_advisory_xact_lock(29999, split_part(p_dest_shard, '_', 4)::int) = false THEN -- N.B. make sure the int constant (29999) you use here is unique within your system RETURN; END IF; EXECUTE format($insert$ INSERT INTO %2$I ( site_id, ingest_time, request_count, success_count, error_count, average_response_time_msec ) SELECT site_id, date_trunc('minute', ingest_time) as minute, COUNT(1) as request_count, SUM(CASE WHEN (status_code between 200 and 299) THEN 1 ELSE 0 END) as success_count, SUM(CASE WHEN (status_code between 200 and 299) THEN 0 ELSE 1 END) as error_count, SUM(response_time_msec) / COUNT(1) AS average_response_time_msec FROM %1$I WHERE date_trunc('minute', ingest_time) > (SELECT COALESCE(max(ingest_time), timestamp '10-10-1901') FROM %2$I) AND date_trunc('minute', ingest_time) < date_trunc('minute', now()) GROUP BY site_id, minute ORDER BY minute ASC; $insert$, p_source_shard, p_dest_shard); END; $$ LANGUAGE 'plpgsql';
Inside this function you can see the dashboard query from earlier. It’s been wrapped in
some machinery which writes the results into
http_request_1min and allows passing in
the name of the shards to read and write from. It also takes out an advisory lock, to
ensure there aren’t any concurrency bugs where the same rows are written multiple times.
The machinery above which accepts the names of the shards to read and write is necessary because only the master has the metadata required to know what the shard pairs are. It has its own function to figure that out:
-- this function is created on the master CREATE FUNCTION colocated_shard_placements(left_table REGCLASS, right_table REGCLASS) RETURNS TABLE (left_shard TEXT, right_shard TEXT, nodename TEXT, nodeport BIGINT) AS $$ SELECT a.logicalrelid::regclass||'_'||a.shardid, b.logicalrelid::regclass||'_'||b.shardid, nodename, nodeport FROM pg_dist_shard a JOIN pg_dist_shard b USING (shardminvalue) JOIN pg_dist_shard_placement p ON (a.shardid = p.shardid) WHERE a.logicalrelid = left_table AND b.logicalrelid = right_table; $$ LANGUAGE 'sql';
Using that metadata, every minute it runs a script which calls
rollup_1min once for
each pair of shards:
#!/usr/bin/env bash QUERY=$(cat <<END SELECT * FROM colocated_shard_placements( 'http_request'::regclass, 'http_request_1min'::regclass ); END ) COMMAND="psql -h \$2 -p \$3 -c \"SELECT rollup_1min('\$0', '\$1')\"" psql -tA -F" " -c "$QUERY" | xargs -P32 -n4 sh -c "$COMMAND"
There are many ways to make sure the function is called periodically and no answer that
works well for every system. If you’re able to run cron on the same machine as the
master, and assuming you named the above script
run_rollups.sh, you can do something
as simple as this:
* * * * * /some/path/run_rollups.sh
The dashboard query from earlier is now a lot nicer:
SELECT request_count, success_count, error_count, average_response_time_msec FROM http_request_1min WHERE site_id = 1 AND date_trunc('minute', ingest_time) > date_trunc('minute', now()) - interval '5 minutes';
Expiring Old Data¶
The rollups make queries faster, but we still need to expire old data to avoid unbounded storage costs. Once you decide how long you’d like to keep data for each granularity, you could easily write a function to expire old data. In the following example, we decided to keep raw data for one day and 1-minute aggregations for one month.
-- another function for the master CREATE FUNCTION expire_old_request_data() RETURNS void AS $$ SET LOCAL citus.all_modification_commutative TO TRUE; SELECT master_modify_multiple_shards( 'DELETE FROM http_request WHERE ingest_time < now() - interval ''1 day'';'); SELECT master_modify_multiple_shards( 'DELETE FROM http_request_1min WHERE ingest_time < now() - interval ''1 month'';'); END; $$ LANGUAGE 'sql';
The above function should be called every minute. You could do this by adding a crontab entry on the master node:
* * * * * psql -c "SELECT expire_old_request_data();"
That’s the basic architecture! We provided an architecture that ingests HTTP events and then rolls up these events into their pre-aggregated form. This way, you can both store raw events and also power your analytical dashboards with subsecond queries.
The next sections extend upon the basic architecture and show you how to resolve questions which often appear.
Approximate Distinct Counts¶
A common question in http analytics deals with approximate distinct counts: How many unique visitors visited your site over the last month? Answering this question exactly requires storing the list of all previously-seen visitors in the rollup tables, a prohibitively large amount of data. A datatype called hyperloglog, or HLL, can answer the query approximately; it takes a surprisingly small amount of space to tell you approximately how many unique elements are in a set you pass it. Its accuracy can be adjusted. We’ll use ones which, using only 1280 bytes, will be able to count up to tens of billions of unique visitors with at most 2.2% error.
An equivalent problem appears if you want to run a global query, such as the number of unique ip addresses which visited any of your client’s sites over the last month. Without HLLs this query involves shipping lists of ip addresses from the workers to the master for it to deduplicate. That’s both a lot of network traffic and a lot of computation. By using HLLs you can greatly improve query speed.
First you must install the hll extension; the github repo has instructions. Next, you have to enable it:
-- this part must be run on all nodes CREATE EXTENSION hll; -- this part runs on the master ALTER TABLE http_request_1min ADD COLUMN distinct_ip_addresses hll;
When doing our rollups, we can now aggregate sessions into an hll column with queries like this:
SELECT site_id, date_trunc('minute', ingest_time) as minute, hll_add_agg(hll_hash_text(ip_address)) AS distinct_ip_addresses FROM http_request WHERE date_trunc('minute', ingest_time) = date_trunc('minute', now()) GROUP BY site_id, minute;
Now dashboard queries are a little more complicated, you have to read out the distinct
number of ip addresses by calling the
SELECT request_count, success_count, error_count, average_response_time_msec, hll_cardinality(distinct_ip_addresses) AS distinct_ip_address_count FROM http_request_1min WHERE site_id = 1 AND minute = date_trunc('minute', now());
HLLs aren’t just faster, they let you do things you couldn’t previously. Say we did our rollups, but instead of using HLLs we saved the exact unique counts. This works fine, but you can’t answer queries such as “how many distinct sessions were there during this one-week period in the past we’ve thrown away the raw data for?”.
With HLLs, this is easy. You’ll first need to inform Citus about the
aggregate function and its semantics. You do this by running the following:
-- this should be run on the workers and master CREATE AGGREGATE sum (hll) ( sfunc = hll_union_agg, stype = internal, );
Now, when you call SUM over a collection of HLLs, PostgreSQL will return the HLL for us. You can then compute distinct ip counts over a time period with the following query:
SELECT hll_cardinality(SUM(distinct_ip_addresses)) FROM http_request_1min WHERE ingest_time BETWEEN timestamp '06-01-2016' AND '06-28-2016';
You can find more information on HLLs in the project’s GitHub repository.
Unstructured Data with JSONB¶
Citus works well with Postgres’ built-in support for unstructured data types. To demonstrate this, let’s keep track of the number of visitors which came from each country. Using a semi-structure data type saves you from needing to add a column for every individual country and ending up with rows that have hundreds of sparsely filled columns. We have a blog post explaining which format to use for your semi-structured data. The post recommends JSONB, here we’ll demonstrate how to incorporate JSONB columns into your data model.
First, add the new column to our rollup table:
ALTER TABLE http_request_1min ADD COLUMN country_counters JSONB;
Next, include it in the rollups by adding a clause like this to the rollup function:
SELECT site_id, minute, jsonb_object_agg(request_country, country_count) FROM ( SELECT site_id, date_trunc('minute', ingest_time) AS minute, request_country, count(1) AS country_count FROM http_request GROUP BY site_id, minute, request_country ) AS subquery GROUP BY site_id, minute;
Now, if you want to get the number of requests which came from america in your dashboard, your can modify the dashboard query to look like this:
SELECT request_count, success_count, error_count, average_response_time_msec, country_counters->'USA' AS american_visitors FROM http_request_1min WHERE site_id = 1 AND minute = date_trunc('minute', now());